(1) Field of the Invention
The present invention relates to a new process for preparing polyalkyl perylenes. It also relates to new products obtained by this process, i.e. polyalkyl perylenes and organic materials, which are derived therefrom and which possess remarkable electron spin resonance properties.
(2) Description of the Prior Art
The radical cation salts derived from certain condensed polyaromatic hydrocarbons, such as fluoranthene (FA) and perylene (PY), have remarkable electron spin resonance (ESR) properties. For example, a remarkably narrow ESR signal was observed at room temperature with (FA).sub.2,PF.sub.6.
This material has a field absorption signal derivative peak-peak width which amounts to a mere 15 milligauss. This signal is thus virtually 100 times narrower than the ESR signal of diphenylpicrylhydrazyl (DPPH), generally used as field marker in ESR. Although the absence of heteroatoms in these materials can be favorable to the narrowing of the ESR lines, on the other hand, this factor leads to a certain thermal instability. Thus for example, salts prepared from naphthalene are only stable beneath -40.degree. C., while the width of the corresponding ESR line is the narrowest (4 milligauss), amongst this whole series of materials.
The remarkable narrowness of these electron spin resonance lines should enable these materials to be used for realizing highly sensitive magnetometers, provided their thermal stability in time, presently quite inadequate, was significantly improved.
To remedy these deficiencies, the invention proposes new materials prepared from polyalkyl perylenes, which can replace advantageously prior art organic materials, such as those derived from fluoranthene, for the realization of ESR magnetic probes. To synthetize these new products, it was necessary to resort to a novel preparation process. This process makes use of a cyclodehydrogenation reaction of the corresponding 1,1'-dinaphthalene derivatives, initiated by metallic lithium. Compared with prior art processes, this invention offers the advantages of simplicity and of a higher yield.